Observing Constraints and Classification Rules
General Observing Constraints
Every requested observation has multiple observing constraints. Typical observing constraints are:
- the allowable brightest lunar phase
- the allowable smallest moon-to-object angular separation
- the allowable maximum airmass
- the allowable maximum image size (i.e. FWHM at observed wavelength, 'image quality' )
- the allowable sky transparency
- for CRIRES, NACO and SINFONI, the Strehl ratio on the reference star (as applicable).
- for instruments observing in the mid-IR (CRIRES and VISIR), the allowable maximum Precipitable Water Vapour (PWV)
- the allowable twilight constraint that defines the earliest time in minutes with respect to the end of the astronomical twilight when the execution of the OB can be started
- the allowable absolute time window (i.e. for time critical events, multi-epoch monitoring)
- the allowable local sidereal time range (e.g. for ADI observation)
- for VLTI instruments, the availability of the desired baseline
The Observing Constraints are specified by the user at Phase 2 for each Observation Block. Since the execution conditions required by each programme are an important ingredient in the process of building up the Long Term Schedule of an observing semester, and thus determines which programmes can or cannot be scheduled, users are not allowed to specify at Phase 2 constraints that are more strict than those specified in the original proposal. Users can however relax the constraints during the submission of their Phase 2 material. The values in the OB constraint sets that are selected (and approved) during Phase 2 preparation (and review) cannot be changed later during the observing period.
General Classification Rules
Quality Control of OBs executed in Service Mode will be based on the user's specified constraints for airmass, atmospheric transparency, image quality/seeing, moon constraints, as well as Strehl ratio for Adaptive Optics mode observations. If all constraints are fullfilled the OB will get assigned Quality Control grade "A", while the "B" quality control is assigned if some constraint is up to 10% violated. The observations with quality control grades A or B are completed, while those with quality control grade "C" (out of constraints) will be re-scheduled and may be repeated. In exceptional cases an OB may get status completed with quality grade "D", meaning that it is taken out of constraints but will not be repeated.
Note: for most instruments the image quality constraint as defined in the OB is judged against the full width at half maximum (FWHM) of a point source in the resulting image (or spectral image). For the instruments where the image quality cannot be directly measured (AO, VLTI, fibre instrument), it is either not used for classification or is obtained from the wavefront sensor of the active optics of the telescope.